Aluminum oxide substitution into the silicon nitride crystal structure has been known for a long time. An early work on this subject is due to Oyama and Kamigaito entitled "Solid Solubility Of Some Oxides In Si.sub.3 N.sub.4 ", Japan J. Appl. Phys. 10, 1637 (1971). They hot pressed mixtures of aluminum oxide and silicon nitride and found that a new phase was formed with expanded lattice parameters. These results, taken with "comparatively good agreement" with expected densities based on the crystal structure expansion, led these authors to conclude that "the new phase is a solid solution of Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3." Oyama et al worked in the range of about 5 to about 16% by weight Al.sub.2 O.sub.3. In 1975, these authors were awarded U.S. Pat. No. 3,903,230 in which is claimed a method of making these solid solutions from Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 and/or AlN, the composition of the solid solution ranging from about 15% to about 87 % by weight Al.sub.2 O.sub.3. Later, in 1976, Jack and Wilson were awarded U.S. Pat. No. 3,991,166 in which a ceramic material consisting of at least 90% of a silicon aluminum oxynitride solid solution was claimed, as well as methods for making it. The solid solutions all these workers were making have come to be known as sialons.
The above materials are not satisfactory for the purpose of ceramic cutting tools. It was later found that at least one modifying compound which combines with the SiO.sub.2 impurity on the Si.sub.3 N.sub.4 was essential to increase the strength and toughness of the material. This is true for the Si.sub.3 N.sub.4 based cutting tool compositions described in U.S. Pat. No. 4,401,617 as well as for the sialon-based compositions described in U.S. Pat. 4,127,416.
For Si.sub.3 N.sub.4 -based compositions, U.S. Pat. No. 4,401,617 claims a method for making a cutting tool in which about 7% to about 9% of the composition is Y.sub.2 O.sub.3. This method allows for up to about 2.5% by weight of Al.sub.2 O.sub.3, WC, TiC, and Si.sub.3 N.sub.4 which is added by attrition of the milling media.
For sialon-based compositions, (Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3 -AlN solid solutions), U.S. Pat. No. 4,127,416 claims a method of producing a "ceramic product" in which the modifying compound R.sub.2 O.sub.3 (R=Y, Sc, Ce, or La) is added in the range of from about 0.1% to about 10% of the sialon product. The Al.sub.2 O.sub.3 range covers from about 6% to about 20% by weight.
U.S. Pat. No. 4,280,973 discloses a method for making Si.sub.3 N.sub.4 based cutting tools by the cold press and sinter method, a method which for years has been common practice in the cemented carbide cutting tool industry. This method allows for the primary modifying compounds Y.sub.2 O.sub.3, CeO.sub.2, MgO, and ZrO.sub.2 and no more than about 4% by weight Al.sub.2 O.sub.3, WC, WSi.sub.2, W, or TiC added by attrition of the grinding media.
This invention relates to a silicon nitride based cutting tool containing aluminum oxide, yttrium oxide and silicon dioxide.